docs(design): actor pattern for Generator/Sender/Receiver

[oferchen]

Summary

• Document the actor-pattern question (Add parity test for sparse with preallocate #2136) for the Generator, Sender, and Receiver roles.
• Inventory the current threading model (one OS thread per direction plus one disk-commit thread, SPSC channels with zero syscalls, threshold-gated rayon for stat / signature / metadata).
• Compare actor refactor (typed messages, supervised lifecycle, async cooperative tasks) against the current rayon plus dedicated threads plus SPSC topology.
• Re-anchor the analysis on the wire-protocol single-threaded constraint already documented for task Cache CLI supported protocol list formatting #1197 (docs/architecture/parallelization.md, docs/audits/async-ssh-transport.md).
• Recommend an actor-as-feature surface behind --features async-pipeline (multi-host fan-out, fault-injection tests) rather than wholesale replacement of the production hot path.

Test plan

• Markdown renders cleanly on GitHub.
• Cross-references to docs/architecture/parallelization.md, docs/audits/async-ssh-transport.md, docs/audits/tokio-dependency-boundary-2026.md, docs/design/async-migration-plan.md, docs/design/async-channel-abstraction.md, docs/design/arc-wrapped-worksender-multi-producer.md, and docs/design/multi-producer-workqueue.md resolve.
• Source-line citations (`generator/transfer.rs:696`, `receiver/transfer/pipeline.rs:38`, `disk_commit/thread.rs:47-64,172-234`, `pipeline/spsc.rs:1-120`) match the current tree.